1. Field of the Invention
This invention relates to a substrate-supporting device to support a semiconductor substrate within a semiconductor-processing device and, in particular, to the structure of a susceptor device that supports a substrate within plasma CVD equipment.
2. Description of the Related Art
FIG. 1 schematically illustrates conventional plasma CVD equipment. Former plasma CVD equipment comprises a reaction chamber (2), a showerhead (5) connected to a radio frequency oscillator (7), a susceptor (3) carrying semiconductor substrates (10), a heating block (4) within which a heating element (11) is embedded, and an exhaust port (9) connected to a vacuum pump (not shown). Flow-controlled reaction gas is introduced from a gas intake port (6) to the showerhead (5) and flows toward an oncoming semiconductor substrate. The inside of the reaction chamber (2) is maintained at a fixed pressure by a vacuum pump. In addition, the temperature of semiconductor substrates placed on the susceptor (3) is detected by a thermo-couple (12) and is maintained at a fixed temperature with a temperature controller (8). The susceptor (3) is electrically grounded (13) through the heating block, and forms one side of a radio frequency grounded electrode. Thus, a radio frequency plasma electrical discharge is generated in the reaction space between the showerhead (5) and the susceptor (3), reaction gas molecules are ionized and change to plasma near the upper part of the semiconductor substrate (10). As a result, active ion molecules adhere to the semiconductor substrate surface to form a thin film.
Incidentally, susceptors used for the plasma CVD equipment are roughly divided into two types. One type is a susceptor integrated with a reaction chamber or a heater, and another is a susceptor and a heating block which are unintegrated and secured together tightly with a bolt.
An example of the former, as stated in U.S. Pat. No. 5,039,388, is a susceptor comprised of aluminum or aluminum alloy with an anodized surface, which is manufactured as an integrated part with a heater. In addition, there is another type for which a heating part and a substrate-supporting part are manufactured in an integrated structure by embedding both the electrode and the heater wire in aluminum nitride.
As an example of the latter, a susceptor and a heating block are secured tightly together by fastening the susceptor over the heating block with bolts, wherein female screws are formed in the heating block and the bolts are screwed in the female screws through the susceptor. The bolt used here is comprised of Inconel, nickel and stainless steel, which are excellent in heat resistance and corrosion resistance. In addition, as stated in U.S. Pat. No. 5,633,073, there is a susceptor with a structure that uses a metal layer such as molybdenum embedded in an aluminum nitride substrate, which is a ceramic material, as a radio frequency grounded electrode, and wherein the back of the electrode is thread-cut as a female screw to secure the susceptor tightly to a reaction chamber with a metal screw.
However, the susceptors mentioned above have the following drawbacks:
First of all, in the case of aluminum with an anodized surface, it is excellent in resistance to plasma etching, but there is a possibility that a crack may develop on the susceptor surface because of changes caused by the passing of time. To prevent contamination from impurities, cleaning maintenance must be conducted regularly by dismantling the susceptor.
In this case, for a susceptor integrated with a reaction chamber and a susceptor integrated with a heater, it takes quite a lot of working hours, hence the use of a device has to be suspended for long hours and this directly causes lowered productivity.
Also, in the case of another integrated type in which both the electrode and the heater wire are embedded in aluminum nitride, regular cleaning maintenance is required to prevent contaminants from entering from the outer environment, including when carrying a wafer substrate in and out, and it is unavoidable to suspend the use of a device for long hours and to lower productivity. In addition, the generally high manufacturing costs of these integrated susceptors are also problematic.
On the other hand, for the type where a susceptor and a heating block are secured tightly together with bolts, poor adherence may occur due to loosening of the bolts caused by thermal expansion and contraction. As a result, a problem such as deviation of the substrate temperature of a semiconductor substrate from the fixed temperature occurs, affecting process stability. Also, at the time of plasma etching, there is a possibility that bolts made of nickel alloy may corrode when directly exposed to a fluorine-active source. As a result, there is a risk that contaminants resulting from corrosion may cause impurity contamination.
Furthermore, for the type with a metal embedded in an aluminum nitride substrate, which is a ceramic material, and with its back secured tightly to a reaction chamber with screws, there is a risk that this structure may be broken due to different coefficients of thermal expansion between aluminum nitride and a nickel screw if secured too tightly. In addition, there are possibilities that an opening is damaged or a screw becomes loose when a metal screw cannot withstand repeated sudden heating.